Monitoring of solid matter containing liquids in industrial processes can be carried out off-line or on-line. Off-line methods often involve batch sampling and laboratory analyses. They have the benefit of providing accurate and versatile information on the suspension but suffer from considerable time delays.
On-line methods, on the other hand, provide instant or almost instant information on the suspension, but the data that can be obtained is not as accurate as can be achieved in the laboratory. Some suspension properties cannot be measured using present on-line techniques.
An example of a remarkable area where measurements of solid matter containing liquids is needed is forest industry, in which wood pulp samples or filtrates, such as e.g. wire water or thickener filtrates, need to be monitored in order to be able to control the overall process. Further, e.g. solid matter containing liquids of oil and mining industry and water treatment industry, especially water reuse, desalination process, especially membrane processes, and cooling water treatment are of interest to be measured. Many such suspensions include particles, whose amount and size distribution have a considerable effect on upcoming process stages. E.g. agglomeration has, in fact, been shown to be the main threat for deposition and related runnability problems on paper machines. However, wood pulp and pulp industry originating liquids and filtrates have a strong tendency to flocculate, which makes the analysis of the solid matter therein challenging.
Some prior art pulp sample or filtrate monitoring techniques have utilized sample fractionation e.g. by filtration, centrifugation, sedimentation or column flow. The only known continuous fractionator is a column flow fractionator, also called a “tube fractionator”. Tube fractionators are discussed e.g. in WO 2007/22289 and WO 2010/16030.
To date only the so-called flow cytometry technique has shown to be successful in detecting and assessing e.g. particle counts, size and/or type in pulp samples or filtrates originating from pulp and paper making industry. However, that technique is quite sophisticated and requires manual sample pretreatment in the laboratory before measurement. In addition, it cannot be used for online measurements. The advantage of flow cytometry measurements is that the particles in the solid matter containing liquid samples are very comprehensively characterized whereby also disturbing substances can be detected.
On the other hand, there are some lightweight techniques which provide on-line information on the level of amount of small particles in e.g. overall turbidity of samples. However, such information is not sufficient for all process control needs as the methods cannot differentiate different types of particles based, e.g. on hydrophobicity, particle size, and/or nature of the particles, whereby no detailed information is provided on disturbing substances. Such methods are discussed e.g. in WO 2012010744 and WO 2012010745.
Field flow fractionation (FFF) represents an approach in measurement of particles in non-industrial process samples. FFF was first described by J. C. Giddings in 1966 allows for physically separating particles having different physical properties from each other in a suspension. In FFF, a sample is injected to the FFF cell where the particles are subjected to a field e.g. temperature, electricity, gravitation, hence the particles in the sample sediment. A flow of liquid is passed through the cell perpendicular to the sedimentation field and as a result smaller (lighter) particles move faster in the flow direction compared to larger (heavier) particles. In a flow cell, particles travel in a laminar flow and heavy particles sediment faster than light particles and therefore heavy particles experience extra friction upon touching the flow cell walls compared to light particles. There are many different FFF systems available depending on the application and most notably on the particle size range one wants to fractionate. For example, there are sedimentation FFF (SdFFF) systems available where the gravitational field is induced through centrifugal force.
In normal FFF the Z-dimension of the cell is in the range of 100-500 μm. For paper pulp samples or filtrates thereof, these dimensions are way too small to achieve any notable separation. It is also typical that an SdFFF system is only capable of handling very small quantities of sample, which is below of what is needed for a paper mill sample as long as turbidity is used as the primary detector. The main problem with samples originating from industrial processes, e.g. with paper mill samples is the presence of fibers and especially fiber fines that have a strong tendency to flocculate in the FFF cell and thus block the cell. This makes the fractionation challenging as the flocks entrap also light particles.
Thus, prior art methods are unsuitable for separating light particles from heavier ones in many industrially important samples.
In addition to flocculation, another problem is the mechanical or chemical sticking of substances to each other and attaching of stickies and hydrophobic substances to surfaces of known fractionation systems, in particular those based on cross-flow filters or known FFF techniques. Hence, there exists a need for improved fractionation and analysis techniques for example for filtrates or pulp samples. A particular need exists for techniques which would additionally allow continuous on-line monitoring of water-intensive processes.